The present invention relates to well logging tools, and more particularly to induction logging methods and apparatus for measuring the resistivity (or its inverse, conductivity) of earth formations penetrated by a borehole.
The basic principles and techniques for induction logging of earth formations are well known. In brief, the resistivities of the various formation structures are determined by inducing eddy currents to flow in the formations in response to an AC transmitter signal, and then measuring a phase component in a receiver signal generated by the eddy currents. Usually the component of the receiver signal which is in phase with the transmitter signal is taken as indicative of the formation conductivity. With proper coil design, the output signal can be directly and linearly proportional to the electrical conductivities of the formations over most formation conductivity values commonly encountered. The output signal is then multiplied by an appropriate tool constant for recording at the surface as a function of the depth of the tool in the borehole.
An induction logging problem which has received considerable attention has to do with the undesirable appearance of "horns" on many logs at bed boundaries. These horns, which generally appear as narrow spike-like features on the log, are artifacts of the logging tool (particularly noticeable with highly focused tools), not of the formation. The magnitude of the horn effect varies with the dip angle of the formations, increasing non-linearly with increasing dip angles. Compensating for such horns can be very difficult, especially since information about formation dip relative to the borehole is often not known. Even when specific dip information is available, it would certainly be better to have a tool which was not adversely affected in the first place by dipping bed boundaries.
In fact, any asymmetry in the formation structure can cause horns or a horn-like distortion in the log (as can a wash-out, or cave effect, as well). When the asymmetry is more broadly distributed than the sharp transition found at a dipping bed boundary, the distortion in the log may even go unnoticed. This can occur, for example, with the so-called "shoulder effect", which is effectively averaged over a large enough distance that no "horn"as such is seen, but a distortion is still present. Therefore, not only is it desirable to have an induction logging tool which is unaffected by formation asymmetries, but it is also desirable to have an induction logging tool which can further provide an indication or measure of the actual asymmetry of the formation structure itself.
The reason that such asymmetries adversely affect induction logging tools has to do with the way an asymmetrical formation structure distorts or "skews" the transmitter field structure. The resultant "skew" signal then appears on one or more axes orthogonal to the transmitter. In order to describe the skew signal, it is helpful first to review the standard configuration for induction logging wherein the skew signal is absent. Ideal conditions are defined as a sonde centered in the borehole with the borehole axis coincident with the tool mandrel. Moreover, all of the bedding planes are horizontal (i.e., zero dip or deviation angle) and, consequently, orthogonal to the borehole wall and invaded zones. In this instance, all of the eddy currents induced by the sonde are tangential to the boundaries separating media of different conductive properties. The current (field) pattern is axisymmetric and the skew signal is absent. In the parlance of wave guides, the electromagnetic fields are described as "transverse electric" (TE) type fields. That is, the electric fields are transverse to the borehole axis, thereby precluding any longitudinal component of the electric field parallel to tho direction of the borehole axis.
On the other hand, whenever the symmetry of the field pattern is disturbed due to tilt, eccentering, dipping beds, etc., the eddy currents intersect the boundaries and the fields develop a "transverse magnetic" (TM) field component. As described more fully herein, this gives rise to the skew signal.
A need therefore remains for induction logging methods and apparatus which automatically compensate for the presence of asymmetrical formation structures and generate a log of formation conductivity or resistivity which is essentially free of horns at bed boundaries, and free of analogous distortions in other asymmetrical formation structures. A need also remains for such induction logging methods and apparatus which can advantageously furnish an indication or measure of the actual asymmetry of the formation structures being logged. Desirably, such methods and apparatus will also be able to provide information such as the dip and strike of dipping formation beds.